Part 4 (1/2)

FIGURE 51. Foot of the second pair (”second pair of gnathopoda”) of the female of Orchestia Tucurauna, magnified 15 diam.)

For example, the younger s.e.xually mature males of Orchestia Tucurauna, n. sp., have slender inferior antennae, with the joints of the flagellum not fused together, the clasping margin (”palm,” Sp. Bate) of the hand in the second pair of feet is uniformly convex, the last pair of feet is slender and similar to the preceding. Subsequently the antennae become thickened, two, three, or four of the first joints of the flagellum are fused together, the palm of the hand acquires a deep emargination near its inferior angle, and the intermediate joints of the last pair of feet become swelled into a considerable incra.s.sation. No museum-zoologist would hesitate about fabricating two distinct species, if the oldest and youngest s.e.xually mature males were sent to him without the uniting intermediate forms. In the younger males of Orchestia Tucuratinga, although the microscopic examination of their testes showed that they were already s.e.xually mature, the emargination of the clasping margin of the hand (represented in Figure 50) and the corresponding process of the finger, are still entirely wanting. The same may be observed in Cerapus and Caprella, and probably in all cases where hereditary s.e.xual differences occur.

(FIGURE 52. Male of a Bodotria, magnified 10 diam. Note the long inferior antennae, which are closely applied to the body, and of which the apex is visible beneath the caudal appendages.)

Next to the extensive sections of the Stalk-eyed and Sessile-eyed Crustacea, but more nearly allied to the former than to the latter, comes the remarkable family of the Diastylidae or c.u.macea. The young, which Kroyer took out of the brood-pouch of the female, and which attained one-fourth of the length of their mother, resembled the adult animals almost in all parts. Whether, as in Mysis and Ligia, a transformation occurs within the brood-pouch, which is constructed in the same way as in Mysis, is not known.* (* A trustworthy English Naturalist, Goodsir, described the brood-pouch and eggs of c.u.ma as early as 1843. Kroyer, whose painstaking care and conscientiousness is recognised with wonder by every one who has met him on a common field of work, confirmed Goodsir's statements in 1846, and, as above mentioned, took out of the brood-pouch embryos advanced in development and resembling their parents. By this the question whether the Diastylidae are full-grown animals or larvae, is completely and for ever set at rest, and only the famous names of Aga.s.siz, Dana and Milne-Edwards, who would recently reduce them again to larvae (see Van Beneden, 'Rech. sur la Fauna littor. de Belgique' Crustacees pages 73 and 74), induce me, on the basis of numerous investigations of my own, to declare in Van Beneden's words; ”Parmi toutes les formes embryonnaires de podophthalmes ou d'edriophthalmes que nous avons observees sur nos cotes, nous n'en avons pas vu une seule qui eut meme la moindre resemblance avec un c.u.ma quelconque.” The ONLY THING that suits the larvae of Hippolyte, Palaemon and Alpheus, in the family character of the c.u.macea as given by Kroyer which occupies three pages (Kroyer, 'Naturh. Tidsskrift, Ny Raekke,' Bd.

2 pages 203 to 206) is: ”Duo antennarum paria.” And this, as is well known, applies to nearly all Crustacea. How well warranted are we therefore in identifying the latter with the former. However, it is sufficient for any one to glance at the larva of Palaemon (Figure 27) and the c.u.macean (Figure 52) in order to be convinced of their extraordinary similarity!) The caudal portion of the embryo in the Diastylidae, as I have recently observed, is curved upwards as in the Isopoda, and the last pair of feet of the thorax is wanting.

Equally scanty is our knowledge of the developmental history of the Ostracoda. We know scarcely anything except that their anterior limbs are developed before the posterior one (Zenker). The development of Cypris has recently been observed by Claus:--”The youngest stages are sh.e.l.l-bearing Nauplius-forms.”

CHAPTER 9. DEVELOPMENTAL HISTORY OF ENTOMOSTRACA, CIRRIPEDES, AND RHIZOCEPHALA.

The section of the Branchiopoda includes two groups differing even in their development,--the Phyllopoda and the Cladocera. The latter minute animals, provided with six pairs of foliaceous feet, which chiefly belong to the fresh waters, and are diffused under similar forms over the whole world, quit the egg with their full number of limbs. The Phyllopoda, on the contrary, in which the number of feet varies between 10 and 60 pairs, and some of which certainly live in the saturated lie of salterns and natron-lakes, but of which only one rather divergent genus (Nebalia) is found in the sea,* have to undergo a metamorphosis.

(* If the Phyllopoda may be regarded as the nearest allies of the Trilobites, they would furnish, with Lepidosteus and Polypterus, Lepidosiren and Protopterus, a further example of the preservation in fresh waters of forms long since extinguished in the sea. The occurrence of the Artemiae in supersaline water would at the same time show that they do not escape destruction by means of the fresh water, but in consequence of the less amount of compet.i.tion in it.) Mecznikow has recently observed the development of Nebalia, and concludes from his observations ”that Nebalia, during its embryonal life, pa.s.ses through the Nauplius- and Zoea-stages, which in the Decapoda occur partly (in Peneus) in the free state.” ”Therefore,” says he, ”I regard Nebalia as a Phyllopodiform Decapod.” The youngest larvae [of the Phyllopoda] are Nauplii, which we have already met with exceptionally in some Prawns, and which we shall now find reproduced almost without exception. The body-segments and feet, which are sometimes so numerous, are formed gradually from before backwards, without the indication of any sharply-discriminated regions of the body either by the time of their appearance or by their form. All the feet are essentially constructed in the same manner and resemble the maxillae of the higher Crustacea.* (*

”The maxilla of the Decapod-larva (Krebslarve) is a sort of Phyllopodal foot” (Claus).) We might regard the Phyllopoda as Zoeae which have not arrived at the formation of a peculiarly endowed abdomen or thorax, and instead of these have repeatedly reproduced the appendages which first follow the Nauplius-limbs.

Of the Copepoda--some of which, living in a free state, people the fresh waters, and in far more multifarious forms the sea, whilst others, as parasites, infest animals of the most various cla.s.ses and often become wonderfully deformed--the developmental history, like their entire natural history, was, until lately, in a very unsatisfactory state. It is true, that we long ago knew that the Cyclopes of our fresh waters were excluded in the Nauplius-form, and that we were acquainted with some others of their young states; we had learnt, through Nordmann, that the same earliest form belonged to several parasitic Crustacea, which had previously pa.s.sed, almost universally, as worms; but the connecting intermediate forms which would have permitted us to refer the regions of the body and the limbs of the larvae to those of the adult animal, were wanting. The comprehensive and careful investigations of Claus have filled up this deficiency in our knowledge, and rendered the section of the Copepoda one of the best known in the whole cla.s.s. The following statements are derived from the works of this able naturalist. From the abundance of valuable materials which they contain I select only those which are indispensable for the comprehension of the development of the Crustacea in general, because, in what relates to the Copepoda in particular, the facts have already been placed in the proper light by the representation of their most recent investigator, and must appear to any one whose eyes are open, as important evidence in favour of the Darwinian theory.* (* I am still unacquainted with Claus' latest and larger work, but no doubt the same may be said of it.)

(FIGURES 53 AND 54. Nauplii of Copepoda, the former magnified 90, the latter 180 diam.)

All the larvae of the free Copepoda investigated by Claus, have, at the earliest period, three pairs of limbs (the future antennae and mandibles), the anterior with a single, and the two following ones with a double series of joints, or branchiae. The unpaired eye, labrum, and mouth, already occupy their permanent positions. The posterior portion, which is usually short and dest.i.tute of limbs, bears two terminal setae, between which the a.n.u.s is situated. The form in this Nauplius-brood is extremely various,--it is sometimes compressed laterally, sometimes flat,--sometimes elongated, sometimes oval, sometimes round or even broader than long, and so forth. The changes which the first larval stages undergo during the progress of growth, consist essentially in an extension of the body and the sprouting forth of new limbs. ”The following stage already displays a fourth pair of extremities, the future maxillae.” Then follow at once three new pairs of limbs (the maxillipedes and the two anterior pairs of natatory feet). The larva still continues like a Nauplius, as the three anterior pairs of limbs represent rowing feet; at the next moult it is converted into the youngest Cyclops-like state, when it resembles the adult animal in the structure of the antennae and buccal organs, although the number of limbs and body segments is still much less, for only the rudiments of the third and fourth pairs of natatory feet have made their appearance in the form of cus.h.i.+ons fringed with setae, and the body consists of the oval cephalothorax, the second, third, and fourth thoracic segments, and an elongated terminal joint. In the Cyclopidae the posterior antennae have lost their secondary branch, and the mandibles have completely thrown off the previously existing natatory feet, whilst in the other families these appendages persist, more or less altered. ”Beyond this stage of free development, many forms of the parasitic Copepoda, such as Lernanthropus and Chondracanthus, do not pa.s.s, as they do not acquire the third and fourth pairs of limbs, nor does a separation of the fifth thoracic segment from the abdomen take place; others (Achtheres) even fall to a lower grade by the subsequent loss of the two pairs of natatory feet. But all free Copepoda, and most of the parasitic Crustacea, pa.s.s through a longer or shorter series of stages of development, in which the limbs acquire a higher degree of division into joints in continuous sequence, the posterior pairs of feet are developed, and the last thoracic segment and the different abdominal segments are successively separated from the common terminal portion.”

(FIGURE 55. Nauplius of Tetrac.l.i.ta porosa after the first moult, magnified 90 diam. The brain is seen surrounding the eye, and from it the olfactory filaments issue; behind it are some delicate muscles pa.s.sing to the buccal hood.)

There is only one thing more to be indicated in the developmental history of the parasitic Crustacea, namely that some of them, such as Achtheres percarum, certainly quit the egg like the rest in a Nauplius-like form, inasmuch as the plump, oval, astomatous body bears two pairs of simple rowing feet, and behind these, as traces of the third pair, two inflations furnished each with a long seta, but that beneath this Nauplius-skin a very different larva lies ready prepared, which in a few hours bursts its clumsy envelope and then makes its appearance in a form ”which agrees in the segmentation of the body and in the development of the extremities with the first Cyclops-stage”

(Claus). The entire series of Nauplius-stages which are pa.s.sed through by the free Copepoda, are in this case completely over-leapt.

A final and very peculiar section of the Crustacea is formed by the two orders of the Cirripedia and Rhizocephala.* (* The most various opinions prevail as to the position of the Cirripedia. Some ascribe to them a very subordinate position among the Copepoda; as Milne-Edwards (1852).

In direct opposition to this notion of his father's, Alph. Milne-Edwards places them (as Basinotes) opposite to all the other Crustacea (Eleutheronotes). Darwin regards them as forming a peculiar sub-cla.s.s equivalent to the Podophthalma, Edriophthalma, etc. This appears to me to be most convenient. I would not combine the Rhizocephala with the Cirripedia, as Liljeborg has done, but place them in opposition as equivalent, like the Amphipoda and Isopoda. The near relations.h.i.+p of the Cirripedia to the Ostracoda is also spoken of, but the similarity of the so-called ”Cypris-like larvae,” or Cirriped-pupae as Darwin calls them, to Cypris is so purely external, even as regards the sh.e.l.l, that the relations.h.i.+p appears to me to be scarcely greater than that of Peltogaster socialis (Figure 59) with the family of the sausages.)

In these also the brood bursts out in the Nauplius-form, and speedily strips off its earliest larva-skin which is distinguished by no peculiarities worth noticing. Here also we find again the same pyriform shape of the unsegmented body, the same number and structure of the feet, the same position of the median eye (which, however, is wanting in Sacculina purpurea, and according to Darwin in some species of Lepas), and the same position of the ”buccal hood,” as in the Nauplii of the Prawns and Copepoda. From the latter the Nauplii of the Cirripedia and Rhizocephala are distinguished by the possession of a dorsal s.h.i.+eld or carapace, which sometimes (Sacculina purpurea) projects far beyond the body all round; and they are distinguished not only from other Nauplii, but as far as I know from all other Crustacea, by the circ.u.mstance that structures which are elsewhere combined with the two anterior limbs (antennae), here occur separated from them.

The anterior antennae of the Copepoda, Cladocera, Phyllopoda (Leydig, Claus), Ostracoda (at least the Cypridinae), Diastylidae, Edriophthalma, and Podophthalma, with few exceptions relating to terrestrial animals or parasites, bear peculiar filaments which I have already repeatedly mentioned as ”olfactory filaments.” A pair of similar filaments spring, in the larvae of the Cirripedia and Rhizocephala, directly from the brain.

(FIGURE 56. Nauplius of Sacculina purpurea, shortly before the second moult, magnified 180 diam. We may recognise in the first pair of feet the future adherent feet, and in the abdomen six pairs of natatory feet with long setae.)

At the base of the inferior antennae in the Decapoda the so-called ”green-gland” has its opening; in the Macrura at the end of a conical process. A similar conical process with an efferent duct traversing it is very striking in most of the Amphipoda. In the Ostracoda, Zenker describes a gland situated in the base of the inferior antennae, and opening at the extremity of an extraordinarily long ”spine.” In the Nauplii of Cyclops and Cyclopsine, Claus finds pale ”sh.e.l.l-glands,”

which commence in the intermediate pair of limbs (the posterior antennae). On the other hand in the Nauplii of the Cirripedia and Rhizocephala the ”sh.e.l.l-glands” open at the ends of conical processes, sometimes of most remarkable length, which spring from the angles of the broad frontal margin, and have been interpreted sometimes as antennae (Burmeister, Darwin) and sometimes as mere ”horns of the carapace”

(Krohn). The connexion of the ”sh.e.l.l-glands” with the frontal horns has been recognised unmistakably in the larvae of Lepas, and indeed the resemblance of the frontal horns with the conical processes on the inferior antennae of the Amphipoda, is complete throughout.* (* In connexion with this it may be mentioned that, in the females of Brachyscelus, in which the posterior antennae are deficient, the conical processes with the ca.n.a.l permeating them are nevertheless retained.)

(FIGURE 57. Pupa of a Balanide (Chthamalus ?), magnified 50 diam. The adherent feet are retracted within the rather opaque anterior part of the sh.e.l.l.

FIGURE 58. Pupa of Sacculina purpurea, magnified 180 diam. The filaments on the adherent feet may be the commencements of the future roots.)

Notwithstanding their agreement in this important peculiarity, the Nauplii of these two orders present material differences in many other particulars. The abdomen of the young Cirripede is produced beneath the a.n.u.s into a long tail-like appendage which is furcate at the extremity, and over the a.n.u.s there is a second long, spine-like process; the abdomen in the Rhizocephala terminates in two short points,--in a ”moveable caudal fork, as in the Rotatoria,” (O. Schmidt). The young Cirripedes have a mouth, stomach, intestine, and a.n.u.s, and their two posterior pairs of limbs are beset with multifarious teeth, setae, and hooks, which certainly a.s.sist in the inception of nourishment. All this is wanting in the young Rhizocephala. The Nauplii of the Cirripedia have to undergo several moults whilst in that form; the Nauplii of the Rhizocephala, being astomatous, cannot of course live long as Nauplii, and in the course of only a few days they become transformed into equally astomatous ”pupae,” as Darwin calls them.

The carapace folds itself together, so that the little animal acquires the aspect of a bivalve sh.e.l.l, the foremost limbs become transformed into very peculiar adherent feet (”prehensile antennae,” Darwin), and the two following pairs are cast off; like the frontal horns. On the abdomen six pairs of powerful biramose natatory feet with long setae have been formed beneath the Nauplius-skin, and behind these are two short, setigerous caudal appendages (Figure 58).

The pupae of the Cirripedia (Figure 57), which are likewise astomatous, agree completely in all these parts with those of the Rhizocephala, even to the minutest details of the segmentation and bristling of the natatory feet;* they are especially distinguished from them by the possession of a pair of composite eyes. (* Compare the figure given by Darwin (Balanidae Plate 30 Figure 5) of the first natatory foot of the pupa of Lepas australis, with that of Lernaeodiscus Porcellanae published in the 'Archiv fur Naturgeschichte' (1863 Taf 3 Figure 5). The sole distinction, that in the latter there are only 3 setae at the end of the outer branch, whilst in the Cirripedia there are 4 on the first and 5 on the following natatory feet, may be due to an error on my part.) Sometimes also traces of the frontal horns seem to persist.* (*